The backhand's different arm geometry produces different lever arm lengths and torque generation mechanics. 

Lever Arm Analysis 
In a backhand, the forearm is partially folded, reducing the effective rotation radius. Since racket head speed equals the effective radius times the angular velocity, a smaller radius at the same angular velocity produces lower racket head speed. To compensate, the wrist snap becomes critical, providing additional angular velocity to maintain power. 

Conservation of Angular Momentum 
With the arm folded, the reduced radius decreases the moment of inertia. This is the same principle as a figure skater pulling in their arms to spin faster: angular momentum is conserved, so a smaller moment of inertia yields faster rotation. For the same applied torque, the backhand achieves faster angular acceleration, making it effective for quick reaction shots near the front court.